Re: [PATCH v3 01/16] kprobes: Add generic kretprobe trampoline handler

[Masami Hiramatsu <mhiramat@kernel.org>]

On Fri, 28 Aug 2020 01:38:44 +0900
Masami Hiramatsu <mhiramat@kernel.org> wrote:

> +unsigned long __kretprobe_trampoline_handler(struct pt_regs *regs,
> +					unsigned long trampoline_address,
> +					void *frame_pointer)
> +{
> +	struct kretprobe_instance *ri = NULL;
> +	struct hlist_head *head, empty_rp;
> +	struct hlist_node *tmp;
> +	unsigned long flags, orig_ret_address = 0;
> +	kprobe_opcode_t *correct_ret_addr = NULL;
> +	bool skipped = false;
> +
> +	INIT_HLIST_HEAD(&empty_rp);
> +	kretprobe_hash_lock(current, &head, &flags);
> +
> +	/*
> +	 * It is possible to have multiple instances associated with a given
> +	 * task either because multiple functions in the call path have
> +	 * return probes installed on them, and/or more than one
> +	 * return probe was registered for a target function.
> +	 *
> +	 * We can handle this because:
> +	 *     - instances are always pushed into the head of the list
> +	 *     - when multiple return probes are registered for the same
> +	 *	 function, the (chronologically) first instance's ret_addr
> +	 *	 will be the real return address, and all the rest will
> +	 *	 point to kretprobe_trampoline.
> +	 */
> +	hlist_for_each_entry(ri, head, hlist) {
> +		if (ri->task != current)
> +			/* another task is sharing our hash bucket */
> +			continue;
> +		/*
> +		 * Return probes must be pushed on this hash list correct
> +		 * order (same as return order) so that it can be popped
> +		 * correctly. However, if we find it is pushed it incorrect
> +		 * order, this means we find a function which should not be
> +		 * probed, because the wrong order entry is pushed on the
> +		 * path of processing other kretprobe itself.
> +		 */
> +		if (ri->fp != frame_pointer) {
> +			if (!skipped)
> +				pr_warn("kretprobe is stacked incorrectly. Trying to fixup.\n");
> +			skipped = true;
> +			continue;
> +		}
> +
> +		orig_ret_address = (unsigned long)ri->ret_addr;
> +		if (skipped)
> +			pr_warn("%ps must be blacklisted because of incorrect kretprobe order\n",
> +				ri->rp->kp.addr);
> +
> +		if (orig_ret_address != trampoline_address)
> +			/*
> +			 * This is the real return address. Any other
> +			 * instances associated with this task are for
> +			 * other calls deeper on the call stack
> +			 */
> +			break;
> +	}
> +
> +	kretprobe_assert(ri, orig_ret_address, trampoline_address);
> +
> +	correct_ret_addr = ri->ret_addr;

Oops, here is an insane code... why we have orig_ret_address *and* correct_ret_addr?
I'll clean this up.

Thanks,

-- 
Masami Hiramatsu <mhiramat@kernel.org>